How many litters/puppies has the mother had? When did she have her last litter (if less than one year, be very wary, as unless she only had one or two puppies last time they really need at least a year between litters). How old is she? (Less than about two and a half years old is a bit young for a slow maturing breed to be bred from). Do they provide insurance cover? Whats the date of the last eye test, whats the hip scores, Have the parents been shown/worked, any major wins/awards? Can they put you in touch with someone who has one of their puppies? Are they keeping any of the puppies (if they say they are not and only have the one left that may be the one they want you to have, and not be showing you the rest).
Do they have other dogs that you can meet
and if so what are the temperaments like. They should be friendly and outgoing. They should look clean and well cared for (might sound obvious, but plenty of people buy a poorly puppy because they feel sorry for it).
Puppies should have a lovely sort of wheaty smell when you pick them up and sniff them (there's nothing on earth like it!) . Have they been microchipped or tattooed, have they been vet checked. Have they been out in the car, if kennel dogs, have they been indoors to experience washing machine, tv. etc. Have they had any vaccinations (if over 9/10 weeks should have had the first one). Will they give you a diet sheet and food for the first few days, will they be happy for you to contact them at any time if you are not sure of anything. Which vet do they use. Can they recommend a good training class?
Probably other things but these are basics, any good breeder will not mind you asking and seeing all their dogs. Another tip, don't decide on a name before you get one, or else you will end up thinking of the puppy as the name you have chosen and will want to bring him home no matter what. At least try to go back a second time to collect him. Enjoy.